166 TROPIC MOVEMENTS 



purposes of orientation l than in most terrestrial flowering plants, while the 

 Bryophyta and certain flowering aquatics occupy an intermediate position 

 in this respect 2 . 



SECTION 35. Methods of Investigating Geotropism. 



The orienting action of gravity only began to be properly understood 

 when Knight showed 3 that centrifugal force exercised a similar orienting 

 action upon seedlings. On a rapidly rotating vertical wheel, for instance, 

 Knight found that the radicle grew outwards, the plumule inwards, both 

 organs curving so as to place themselves parallel to the direction of the 

 orienting force. In this case the disturbing action of gravity is eliminated 

 by the vertical rotation of the wheel, but if the wheel is rotated horizontally 

 the forces of gravity and of centrifugal force act at right angles to one 

 another upon the seedlings, and the ultimate position of the axis is along 

 a resultant line which bisects the angles between the forces if they are 

 equal, but is nearer to the more powerful one when they are unequal. 

 When the wheel is rotated very rapidly the axes of the seedlings grow 

 almost horizontally 4 . 



If a seedling is slowly and steadily rotated in a horizontal or vertical 

 position on a klinostat so that a revolution is performed in three to forty 

 minutes, the position of the plant is continually altered before any inductive 

 stimulating action of gravity can be made manifest 5 . For most plants 

 two to three revolutions per hour are sufficient, for at this rate practically 

 no centrifugal action is exercised, while at the same time neither the shoot 

 nor root has time to make a curvature before its position is reversed. If 

 each rotation takes several hours the slight, continually changing curvature 

 results in the production of a kind of circumnutation 6 . 



1 Cf. Berthold, Jahrb. f. wiss. Bot., 1882, Bd. XII, p. 572. 



2 The unicellular rhizoids of Marchantia are geotropic, but the thallus less so. Mirbel, Mem. 

 de 1'Acad. royale de Paris, 1835, T. XIII, p. 354; Pfeffer, Arb. d. bot. Inst. in Wurzburg, 1871, Bd. 

 I, p. 89. A few facts concerning the Jungermanniaceae are given by Hofmeister, Pflanzenzelle, 

 1867, p. 294; Frank, Die natiirliche wagerechte Richtung von Pflanzenth., 1870, p. 66. On the 

 Muscineae cf. Bastit, Rev. gen. de Bot., 1891, T. Ill, p. 380; Jonsson, Bot. Ztg., 1899, Referate, 

 p. 132. 



3 Knight, Phil. Trans., 1806, I, p. 99. Knight used a water-wheel, and carried out experiments 

 on rotation in both vertical and horizontal planes. The older and newer literature has been collected 

 by Cisielski, Unters. iiber d. Abwartskriimmung d. Wurzel, Dissertation, 1870. The same work 

 without the review of the literature is given in Cohn's Beitragen z. Biologic, 1871, Bd. I, Heft 2. 

 Cf. also Sachs, Arb. d. bot. Inst. in Wurzburg, 1879, Bd. n, p. 209. 



* Cf. Wigand, Bot. Unters., 1854, P- J 49 '> Hofmeister, Jahrb. f. wiss. Bot., 1863, B( *. ill, p. 141. 



5 This term was given by Sachs (Arb. d. bot. Inst. in Wurzburg, 1879, Bd. II, p. 217), who 

 was the first to use this method to any great extent for the elimination of gravity, although Hunter 

 had used it long ago to a limited extent, and also Dutrochet and Wigand. Hunter, Trans. Soc. 

 Imp. med., 1800, Vol. II. See A. P. de Candolle, Pflanzenphysiol., 1835, Bd. II, p. 556. Cf. also 

 F. Darwin, Linnean Soc. Journal, 1881, xvin, p. 425. 



6 Darwin and Pertz. Annals of Botany, 1892, Vol. vii, p. 245 ; 1903, Vol. XVII, p. 93. 



